Recent trends in the access communications market show that data rates up to 100 Mb/s which are provided by Very High Speed Digital Subscriber Line 2 (VDSL2) services using Vectoring as defined in ITU-T Recommendation G.993.5, see ITU-T Rec. G.993.5-2010 Self-FEXT CANCELLATION (Vectoring) for Use with VDSL2 Transceivers, 2010, are not always sufficient. Bit rates up to 1.0 Gb/s are sometimes required. VDSL2 employs vectoring for noise reduction. The VDSL2 service is provided from a street cabinet in a fiber to the curb (FTTC) architecture for distances up to 1000 m.
Such high bit rates is possible with the G.fast service, see ITU-T Rec. G.9701. Fast Access to Subscriber Terminals—Physical layer specification, 2013. The G.fast technology achieves comparably high data rates in fiber to the distribution point (FTTdp) network topologies where the service is provided from a distribution point (DP) which may be as close as 50 m-100 m to the customers.
In some cases, an intermediate step between VDSL2-based FTTC and G.fast-based FTTdp can be done, such as deploying G.fast FTTdp technology from VDSL2 FTTC locations to serve short and medium reach customers with higher bit rates.
In an intermediate step—during roll-out of G.fast service—, the G.fast data rates are decreased to 200-400 MBit/s, but the reach is far beyond the regular FTTdp G.fast reach, which is 250 m. The extended reach G.fast service reaches up to 400 m and thus shall be able to coexist with regular G.fast and with VDSL2 deployed from FTTC. This will allow a gradual replacement of VDSL2 service of the FTTC with high speed FTTdp G.fast services.
Reference implementations of such gradual replacement face certain drawbacks and limitations. E.g., for long-reach G.fast-based transmission, transmission in high-frequency components of the spectrum may be attenuated significantly. On the other hand, during gradual replacement of VDSL2 systems with high speed FFTdp G.fast systems, low-frequency components of the spectrum may be occupied by the VDSL2 system. Thus, the available spectrum for G.fast-based transmission is limited.